arr.flat(Infinity)

arr.join().split(',').map(Number)

const flatten=arr=>{
	const result=[]
	while(arr.some(item=>Array.isArray(item))){
		result.concat(...arr)
	}
	return result
}

const flatten=arr=>arr.reduce((acc,cur)=>(
Array.isArray(cur)?
 [...acc,...flatten(cur)]
 :
 [...acc,cur])
,[])